The invention concerns a control circuit for a hydraulic power lift drive by a displacement pump.
Already known is a control circuit for a hydraulic power lift driven by a displacement pump, having a check valve releasable at least for lowering movements of the power lift, a load-sensing line carrying the load pressure and an actuation means switching the check valve off and on, which controls the flow from a pump line into an operating line leading to the power lift and/or from the power lift into a tank line leading to the tank.
In such a control circuit, there is provided a servo valve which by one control edge actuates the check valve or the shut-off block, via another control edge acts on a hydraulic component which switches the displacement pump feeding a hydraulic system into a variety of operating conditions, by still another control edge with a variable opening surface controls a pump feed flow between a pump line and a consumer line leading to the power lift, in which the shut-off block is seated, and by a fourth control edge with a variable opening area controls a relief oil flow between the service line and a tank line.
This conventional control circuit was required for hydraulic systems fed by constant displacement pumps, since in such systems, in the neutral position of the servo valve the constant displacement pump had to be switched off and the shut-off block closed in order to keep power losses as small as possible. Such a control circuit is likewise used for hydraulic systems which are increasingly operated with variable displacement pumps for further reducing power losses.
The servo valve therefore has a switching and a flow-controlling function, so that there is necessarily a linkage of these two functions in the servo valve. This results in the problem of optimizing the dynamics and the accuracy of the control circuit despite such linkage of the two functions.
For this purpose, a control circuit pursuant is disclosed in DE-OS 3733679. Use of the displacement pump in conjunction with the actuating means permits the switching functions of the control circuit to be separated from the flow-controlling functions in such fashion that a good response behavior of the control circuit is produced, more particularly, the disadvantage of an initially gradual pressure buildup in the consumer line upon its activation is avoided. This control circuit therefore offers opportunities for reacting variably to a variety of operating requirements demanded of the power lift. If accuracy and rapidity of reaction are required, the variable displacement pump is driven in the load-sensing mode and the shut-off block remains open. In operating phases with low demands, for example, when driving a vehicle under load, outfitted with a hydraulic power lift, the variable displacement pump may be switched into so-called standby operation by connection of the load-sensing line to the tank pressure level, where the shut-off block is automatically closed and the load thereby secured. Since the variable displacement pump is always switched to load pressure before the shut-off block opens, at a high load pressure pilot actuation of the shut-off block may alternatively be eliminated. This means that design of the shut-off block may be made much less costly structurally, since various additional hydraulic control lines and elements of the conventional control circuit are no longer required. At the same time, the linkage of load-sensing operation with actuation of the shut-off block without recourse to pilot actuation automatically results in a safety function for controlled and regulated hydraulic systems with a mass load. Since the pilot pressure level is low in comparison to the load pressure, lowering of a heavy load can always take place only in the neutral position of the servo valve. The sudden or brief drop of a heavy load on startup of the control circuit is therefore likewise precluded when, for any reason, the servo valve inadvertently stands or stood in its lowered position. In this connection, it is possible to carry out flow control by the servo valve and the switching function, through which release of the check valve or the shut-off block is effected, by means of a single pilot pressure, a partial region of the pilot pressure spectrum preferably being reserved for the switching function and it being provided that, at a low pilot pressure level, the actuating unit responds as the servo valve.